Wikipedia: German Diaspora
. This article helps give background to when, where, and why Germans left their homelands. The article also lists major subgroups, such as Mennonites and Hessians, including where their major settlements were worldwide.
: This article reinforces being methodical in your research. It reviews the major periods of emigration, ports where the immigrants left from (Bremen, Hamburg, Le Havre, Antwerp, Netherlands, Copenhagen), and where to look for passage records, if they exist.
Links to Immigration/Emigration Databases